Change tensorimpl opencv `future_std::span<cv::Mat>` to `cv::Mat`

Why ? TensorImpl_opencv attribute future_std::span<cv::Mat> mData; is not relevant.

MR Content :

• Change to cv::Mat mData;
• Adapt functions of TensorImpl_opencv
• Adapt tests
• Change the opencv::imread flag from IMREAD_COLOR to IMREAD_UNCHANGED and adpat tests. The flag IMREAD_COLOR was always loading 3 channels, even for grayscale images while IMREAD_UNCHANGED adapts to the input file. See : https://docs.opencv.org/4.x/d8/d6a/group__imgcodecs__flags.html#ga61d9b0126a3e57d9277ac48327799c80

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Implement the solution presented in issue #3 (moved).

• Change the constructor of TensorImpl to take the dimensions of the tensor as argument instead of the number of elements. For tensorImpl_opencv :
• Add mDims attribute in the class to use in Lazy_init (Instanciation of cv::Mat)
• Override the resize function from TensorImpl.hpp to update mDims